1. Field of the Invention
The invention relates to the field of optical storage systems, and in particular, to servo circuitry in an optical storage system for correlating signals in the optical storage system.
2. Statement of the Problem
An optical disk stores data on circular tracks on its surface. An optical disk device reads the optical disk by centering an optical head over a track, reflecting a light beam off of the surface of the disk, and detecting the reflected light beam with a four-quadrant photo-detector. The photo-detector generates a signal for each quadrant and transfers the four signals to servo circuitry. The servo circuitry uses the quadrant signals to keep the optical head centered over the track when following the track to read from the optical disk. The optical disk device is also able to seek out a particular track on the optical disk to read from that track. The optical disk device seeks by counting the number of tracks that the optical head crosses over during the seek. The servo circuitry again uses the quadrant signals to count track crossings and locate the desired track. A problem with the current servo circuitry is if the optical head tilts while following a track or seeking for a track, the servo circuitry may improperly position the optical head due to the manner in which the servo circuitry uses the quadrant signals.
FIG. 1 shows an optical disk system 100 in the prior art. Optical disk system 100 is comprised of an optical disk device 102 coupled to a servo system 104. Optical disk device 102 includes an optical head that contains an optical transducer and a four-quadrant photo-detector 110. Servo system 104 is comprised of a servo detector 106 connected to a servo controller 108. Servo detector 106 is comprised of diagonal signal generators 122-123, a positive correlator 126, a negative correlator 128, an Adaptive Dual Arm Correlator (ADAC) 130, and a track counter 132. Optical disk device 102 is connected to diagonal signal generators 122-123 in servo detector 106. Diagonal signal generators 122-123 are connected to positive correlator 126 and negative correlator 128. Positive correlator 126 and negative correlator 128 are connected to ADAC 130. ADAC 130 is connected to track counter 132 and servo controller 108. Track counter 132 is connected to servo controller 108. The servo controller 108 is connected to optical disk device 102.
Optical disk device 102 stores data on an optical disk in the form of a series of pits arranged into tracks. The data is encoded on pit edges. The pit lengths and the distance between pits are integer channel bit periods. Run-length limited encoding determines the distance between edges. To read from the optical disk, the optical disk device 102 positions the optical head over the optical disk. The optical transducer projects a light beam onto a surface of the optical disk. The light beam reflects off of the pits and the surface of the optical disk, and onto photodetector 110. Photodetector 110 has four quadrants and each quadrant generates a signal. For instance, quadrant B generates a signal B representing the reflection of the light beam captured by quadrant B. If the optical head is centered over a track on the optical disk and the track runs parallel to a line between quadrants A and B, then quadrants A-D will see substantially similar light power and will generate similar amplitude signals. If the optical head is off-center, quadrants A and D see substantially similar light power and quadrants B and C see substantially similar light power. In the off-track case the sum of the power at quadrants A and D is different from the sum of the power at quadrants B and C. Photo-detector 110 transfers respective signals A-D to servo system 104.
Servo system 104 processes embedded tracking error data to center the optical head over the track with logic appreciated by one skilled in the art as follows. Servo system 104 receives signals A-D. Signals A-D include encoded user data and the embedded tracking error data. A read channel circuit (not shown) processes signals A-D to detect the encoded user data. Servo system 104 processes signals A-D to detect the embedded tracking error data. The embedded tracking error data is not physically written onto the optical disk, but is inherent to optical systems and results from the reflection of the light beam changing as the optical head moves off-track. Servo system 104 uses the embedded tracking error data to center the optical head during track following functions and to find a particular track during seek functions.
Within servo system 104, servo detector 106 generates a Position Error Signal (PES) and a track count signal by logic appreciated by one skilled in the art as follows. The PES represents how far off track the optical head is during the track following functions. The track count signal represents track crossings during the seek functions. Diagonal signal generator 122 receives signal A and signal C from photo-detector 110. Diagonal signal generator 122 adds signal A and signal C to generate a diagonal signal S1. Diagonal signal generator 122 transfers S1 to positive correlator 126 and negative correlator 128. Diagonal signal generator 123 receives signal B and signal D from photo-detector 110. Diagonal signal generator 123 adds signal B and signal D to generate a diagonal signal S2. Diagonal signal generator 123 transfers S2 to positive correlator 126 and negative correlator 128.
Positive correlator 126 receives S1 from diagonal signal generator 122 and S2 from diagonal signal generator 123. Positive correlator 126 correlates S1 and S2 by summing S1 and S2 over a length L bits, where L represents a correlation length. Positive correlator 126 generates a positive correlation CorrP and transfers CorrP to ADAC 130. Negative correlator 128 also receives S1 from diagonal signal generator 122 and S2 from diagonal signal generator 123. Negative correlator 128 correlates S1 and S2 by summing S1 and S2 over the length L bits. Negative correlator 126 generates a negative correlation CorrN and transfers CorrN to ADAC 130.
ADAC 130 receives CorrP from positive correlator 126 and CorrN from negative correlator 128. ADAC 130 generates the PES by taking the difference between CorrP and CorrN. ADAC 130 transfers the PES to servo controller 108. ADAC 130 also receives S1 and S2 from diagonal signal generators 122-123. ADAC 130 generates a Phase Offset Signal (POS) by determining the offset between S1 and S2. ADAC 130 transfers the POS to track counter 132. The POS is a sinusoidal signal that is cyclic with track crossings. Track counter 132 receives and filters the POS with a programmable band pass filter. Track counter 132 slices the filtered POS and counts the number of edges to generate the track count signal. Track counter 132 transfers the track count signal to servo controller 108.
Servo controller 108 uses the PES to center the optical head during the track following functions and the track count signal to position the optical head over the particular track during the seek functions.
A problem with servo detector 106 is ADAC 130 is an adaptive algorithm that relies on a measured phase difference between the diagonal signals S1 and S2 to be linear between +/xe2x88x92pi. In practice the phase difference is more apt to be sinusoidal. ADAC 130 has problems track counting and finding a transducer gain for servo system 104 during calibration. Another problem is noise and defects easily corrupt track counter 132. Track counter 132 uses a programmable filter to ameliorate the effects of noise. However, the filter requires the servo detector 106 adjust a bandwidth of the POS according to the seek velocity. Unfortunately, the filter limits the bandwidth of track counter 132 and requires the servo detector 106 adjust a bandwidth of the POS according to the seek velocity.
FIG. 2 shows diagonal signal generator 122 in the prior art. Diagonal signal generator 122 is comprised of band-pass filters 202-203, comparators 212-213, samplers 222-223, delays 232-233, and OR circuit 242. Band-pass filter 202 connects to comparator 212. Comparator 212 connects to sampler 222. Sampler 222 connects to delay 232. Delay 232 connects to OR circuit 242. Band-pass filter 203 connects to comparator 213. Comparator 213 connects to sampler 223. Sampler 223 connects to delay 233. Delay 233 connects to OR circuit 242.
In operation, band-pass filter 202 receives analog signal A from photo-detector 110. Band-pass filter 202 filters signal A to attenuate noise and generates a first filtered signal. Band-pass filter 202 transfers the first filtered signal to comparator 212. Comparator 212 converts the first filtered signal into a first polarity square wave using hysteresis and transfers the first polarity square wave to sampler 222. Sampler 222 samples the first polarity square wave to generate a first binary square wave. Sampler 222 transfers the first binary square wave to delay 232. Delay 232 delays the first binary square wave in order to calibrate servo system 104 according to the characteristics of optical disk device 102. Delay 232 transfers the delayed first binary square wave to OR circuit 242.
Band-pass filter 203 receives analog signal C from the photodetector 110. Band-pass filter 203 filters signal C to attenuate noise and generates a second filtered signal. Band-pass filter 203 transfers the second filtered signal to comparator 213. Comparator 213 converts the second filtered signal into a second polarity square wave using hysteresis and transfers the second polarity square wave to sampler 223. Sampler 223 samples the second polarity square wave to generate a second binary square wave. Sampler 223 transfers the second binary square wave to delay 233. Delay 233 delays the second binary square wave in order to calibrate servo system 104 according to the characteristics of optical disk device 102. Delay 233 transfers the delayed second binary square wave to OR circuit 242.
OR circuit 242 receives the first and second binary square waves from delay 232 and delay 233 respectively. OR circuit 242 ors the first binary square wave and the second binary square wave to generate diagonal signal S1. OR circuit 242 transfers S1 to positive correlator 126 and negative correlator 128. Diagonal signal generator 123 operates the same way on signal B and signal D to generate diagonal signal S2. Diagonal signal generator 123 transfers S2 to positive correlator 126 and negative correlator 128.
A problem with diagonal signal generator 122 is its susceptibility to lens shift. Lens shift occurs when the amplitude of two of the quadrant signals A-D decrease while the other two remain the same. The decrease in the amplitude of two signals is commonly caused by a tilt in a lens in the optical head. The amplitude variation adversely affects the phase of diagonal signals S1 and S2 when diagonal signal generators 122-123 add signals A-D to generate S1 and S2. Lens shift changes the PES""s baseline as a function of radial lens tilt. Lens shift can cause servo system 106 to improperly position the optical head when track following. Lens shift can also cause track counter 132 problems in slicing the POS to generate the track count signal. Delays 232-233 reduce effects of lens shift, but after calibration using a complicated calibration algorithm.
FIG. 3 shows positive correlator 126 in the prior art. Positive correlator 126 is comprised of delays 302-303, xcexa3XNORs 306-307, and an adder 310. Delay 302 connects to xcexa3XNOR 306. xcexa3XNOR 306 connects to adder 310. Delay 303 connects to xcexa3XNOR 307. xcexa3XNOR 307 connects to adder 310.
In operation, delay 302 receives S2 from diagonal signal generator 123 and a predetermined correlation offset xcex94. The correlation offset xcex94 is adaptively adjusted to maximize the positive correlation and the negative correlation. Delay 302 delays S2 by xcex94+ and transfers delayed S2 to xcexa3XNOR 306. xcexa3XNOR 306 receives S1 from diagonal signal generator 122 and delayed S2 from delay 302. xcexa3XNOR 306 sums the XNOR of S1 and delayed S2 over a length L bits, where L represents a correlation length. xcexa3XNOR 306 generates a correlation signal +CorrP and transfers +CorrP to adder 310.
Delay 303 receives S2 from diagonal signal generator 123 and xcex94. Delay 303 delays S2 by xcex94xe2x88x92 and transfers delayed S2 to xcexa3XNOR 307. xcex94xe2x88x92 is slightly smaller than xcex94+. xcexa3XNOR 307 receives S1 from diagonal signal generator 122 and delayed S2 from delay 303. xcexa3XNOR 307 sums the XNOR of S1 and delayed S2 over the length L bits. xcexa3XNOR 307 generates a correlation signal xe2x88x92CorrP and transfers xe2x88x92CorrP to adder 310.
Adder 310 receives +CorrP and xe2x88x92CorrP from the xcexa3XNOR 306 and xcexa3XNOR 307 respectively. Adder 310 adds +CorrP and xe2x88x92CorrP to generate the correlation signal CorrP. Adder 310 transfers CorrP, +CorrP, and xe2x88x92CorrP to ADAC 130. Negative correlator 128 operates substantially in the same manner as positive correlator 126 to generate correlation signals CorrN, +CorrN, and xe2x88x92CorrN. A difference is negative correlator 128 delays S1 by xcex94 instead of delaying S2. The negative correlator 128 transfers CorrN, +CorrN, and xe2x88x92CorrN to ADAC 130. A problem with positive correlator 126 and negative correlator 128 is both are overly complicated.
The solution involves correlating signals based on a correlation data structure in an instantaneous correlator. The instantaneous correlator advantageously correlates four signals using the correlation data structure instead of correlating diagonal signals as in the prior art. The instantaneous correlator is less prone to lens shift errors and is simpler to implement than the prior correlators.
The instantaneous correlator is comprised of a setup system and correlation system. The correlation system contains the correlation data structure. The setup system receives four signals and converts the signals to digital values. The correlation system takes the digital values and enters them into the correlation data structure. The correlation data structure yields a correlation value based on the digital values. The correlation value is advantageously a function of all four signals.